It is well known that muscle contractions can be measured by attaching transducers to the muscle and detecting signals generated by the transducers that are proportional to the distance between the attachment points. One device that has been used for this purpose employs a transducer consisting of an elastic tube which is filled with a conductive fluid. To operate this device, the ends of the tube are sutured in tension to the muscle whose displacement or contraction is to be measured. As the muscle moves, the length of the tube changes and, consequently, the electrical resistance of the tube also changes. Unfortunately, such a device has the disadvantage that it must be sutured to the muscle. Thus, it is not able to respond well to very small displacements, or to displacements which occur within a short period of time. In addition, such devices are more prone to cause injury or trauma to the muscle during their attachment and removal.
Another device, as disclosed in U.S. Pat. No. 3,937,212 to Fletcher et al., pertains to a miniature muscle displacement transducer. It senses muscle displacement or contraction utilizing a curved structural beam of high elastic compliance which is connected at its ends to two prongs. A sensitive strain gauge is bonded to the beam to generate an output that is directly related to changes in the beam curvature. As the muscle under observation expands and contracts, the prongs move and the beam curvature correspondingly experiences changes, which are detected by the strain gauge. No suturing is required with this apparatus. Instead, the pair of elongated prongs, which are oriented substantially parallel to one another and disposed in a plane which is substantially perpendicular to the plane of the curved beam, have sharp tips that are insertable into the muscle. The transducer must be fabricated and calibrated, however, to correlate its output signal changes as a function of changes in beam curvature. Moreover, it measures lateral contractions, and due to parallel positioning of the elongated probes, is not well suited to measure displacement between opposite sides of the muscle, such as is required when measuring the ventricular muscle of the heart.
It is also well known that ultrasonic transducers can be attached to a muscle to measure the variations in distance between the transducers as an indication of the muscle's movement and activity. Such ultrasonic transducers provide increased sensitivity, and can detect very small changes in the relative positions of the transducers within very small increments of time. Thus, ultrasonic transducers are able to provide an effective real-time representation of muscle activity. Unfortunately, conventional methods using such crystal ultrasonic transducers require the tranducers be sutured to the muscle to assure the transducers are properly held in place. In addition, the transducers must be properly aligned so the ultrasound energy is adequately focused between the transmitter and receiver to provide useful and accurate muscle activity information. Other transducers using sonomicrometry (e.g. piezoelectric crystals) to detect variations in heart muscle movement, have attempted to solve the problem of maintaining alignment of the crystal transducers. Some have used various shaped crystals which have larger beam widths and greater sensitivity with which to overcome any misalignment. These devices, however, still require implantation in the epicardium using a securing ring. Such a device is disclosed in the article entitled "An Improved Transducer for Measurement of Cardiac Dimensions with Sonomicrometry" by Trigt et al., American Physiological Society, 1981.
Yet another genre of transducer is disclosed in co-pending U.S. patent application Ser. No. 383,205, assigned to the same assignee as the present invention, which recognizes the need for a sonomicrometry system that can accurately measure heart contractions to monitor performance of the heart during cardiovascular surgery, and to warn immediately when a problem occurs. This invention provides for heart contraction monitoring without requiring suturing and prealignment of the transducers by attaching the sonic transducers to a transducer caliper for attachment to the heart muscle.
With specific regard to the heart muscle, the obvious desirability for relatively small sized transducers, unfortunately, is offset by the relatively low sound pressure levels produceable by these small transducers. Specifically, the generation and processing of meaningful electronic signals from low sound pressure levels is complicated. For example, the sound pressure level of the transmitted ultrasonic signal is on the order of one (1) microvolt. Furthermore, this signal undergoes significant attenuation as it crosses the heart and is occasionally undetected by the receiving transducer. Moreover, the detection problem is compounded by a relatively large amount of sonic background noise in the heart. Consequently, it is often the case that false detection signals are generated by the receiving transducer in response to noise, as opposed to a bona fide measurement transmission. These false signals are preferably rejected during the signal processing in order to present operators with more meaningful data. It will be understood, therefore, that given the inherent complexity of conditioning the relatively weak and often erratic heart measurement signals noted above, a signal processor is desired which can quickly perform relatively complicated signal processing of the signals generated by relatively small sized ultrasonic transducers. Relatively high degrees of processing speed and data flow rate are even more important in light of medically-based requirements for a high degree of heart size measurement accuracy, in addition to the requirement for a relatively large data flow. For example, it can be required that the transducer system suggested above produce 1000 heart size measurements per second, and that each measurement be accurate to within one-tenth of one millimeter. Accordingly, it will be understood that the speed and capacity of digital microprocessors make them attractive as signal processors for the kinds of sonomicrometry devices noted above.
In accordance with the discussion above, the present invention recognizes that a need exists to provide a heart contraction monitor which can process, store, and display a large number of measurements per unit time. The present invention also recognizes that a need exists to provide a heart contraction monitor which can recognize and reject heart size measurement signals that are unreasonably large or small. In addition, the present invention recognizes that a need exists to provide a heart contraction monitor which can control and monitor the pulsing of a sonic transducer system for measuring heart size. Finally, the present invention recognizes that a need exists to provide a heart contraction monitor which is relatively easy to operate and cost effective to manufacture.